cmd/compile: MOVBload and MOVBQZXload are the same op

cmd/compile: MOVBload and MOVBQZXload are the same op

No need to have both ops when they do the same thing.
Just declare MOVBload to zero extend and we can get rid
of MOVBQZXload.  Same for W and L.

Kind of a followon cleanup for https://go-review.googlesource.com/c/19506/
Should enable an easier fix for #14920

Change-Id: I7cfac909a8ba387f433a6ae75c050740ebb34d42
Reviewed-on: https://go-review.googlesource.com/21004
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>